confused newbie: blades for my 7 1/4" worm drive circular saw?

i just picked up a worm drive circular saw (my first circular saw)...along with a nice $29 (ouch) carbide "finishing" blade.
it turns out this saw uses a "DIAMOND" arbor mounting hole..rather than just ROUND. I can swear all the blades i saw @ home depot were ROUND. Ive looked on the net and dont find ANY "worm drive blades."
what is going on here...can regular ROUND blades be mounted on my "DIAMOND" arbor mounting hole somehow? I see a diamond outline traced around the ROUND-type blade i have here in my hand...and it seems like it can be "punched out" ?
Thanks guys! Off to a puzzling start here...
Mohsin
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Yur right, it's a knock out and most (decent) circular saw blades will have them. Set the blade over a hole (just large enough for the diamond to fall into), put an old screwdriver on it and give it a whack with a hammer. You can twist it out from there..
Good luck Rob

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oh ok...i just wasnt sure. Did'nt want to warp the $29 blade. =)
thanks

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